The present invention is generally directed to nuclear magnetic resonance (NMR) imaging systems. More particularly, the present invention is directed to transverse gradient coils for use in NMR imaging systems.
It has been shown in recent years that the phenomenon of nuclear magnetic resonance may be advantageously employed to construct tomographic medical images of the internal human anatomy. While NMR methods have been employed for many years in the past in the field of chemistry to identify various atomic constituents found in material samples, the application of this technology to imaging is relatively new. At present, its medical and diagnostic applications appear to be numerous and significant. NMR imaging methods also appear to be able to provide physicians, diagnosticians and other medical personnel with heretofore unavailable information concerning various human and animal physiological and metabolic processes particularly those involving phosphorus and the utilization of phosphorus-containing compounds.
For proper operation, the apparatus employed must be able to generate several distinct magnetic fields typically within a cylindrical volume into which the patient or object under study is placed. The present invention is directed to the formation of one of these fields. For example, it is necessary to provide a large but constant magnetic field oriented along the axis of the cylindrical patient volume. Magnetic fields from about 0.04 to as high as 1.5 tesla or more are contemplated as comprising this constant magnetic field. Additionally, superimposed upon the large constant field in the longitudinal or z-direction there is a much smaller z-gradient field for the purpose of providing position information in the longitudinal direction, that is in the z-axis direction. To provide position information in a plane perpendicular to the cylindrical axis, two additional magnetic fields are provided. One is called the "x gradient" and provides a field B such that .differential.B.sub.z /.differential.x is constant. The other is called the "y gradient" and provides a field B such that .differential.B.sub.z /.differential.y is constant. The x and y directions are perpendicular to one another, as in the conventional Cartesian coordinate system. Furthermore, the transverse magnetic fields which are provided by coils for these purposes, must produce a magnetic field whose component in the z direction varies in a linear fashion across the transverse direction of the sample object being studied. It is an object of the present invention to provide winding patterns which are particularly useful for providing such transverse gradient magnetic fields.
Transverse gradient coil designs for NMR applications are currently based on designs directed to chemical evaluation of small samples rather than the construction and design of large scale NMR systems for medical diagnosis. For example, in U.S. Pat. No. 3,622,869 issued 1971 to M. J. E. Golay and titled "High Homogeneity Coils for NMR Apparatus," there is described a saddle-type coil structure exhibiting the desired degree of linearity only in a restricted central region of the cylindrical coil form. Similarly, in a paper presented by J. Dadok at the Tenth Annual Experimental NMR Conference in Pittsburgh, Pa. in 1969 titled "Shim Coils for Superconducting Solenoids," similar coils were described. These coils are deficient in that the magnetic fields produced by them do not have the properties desired for medical diagnostic applications.
Although they are satisfactory for samples that are small compared to the cylindrical form on which the coils are wound, they are not useful if the sample is large enough to approach the walls of the cylindrical coil form. This is the situation in NMR imaging. A particular drawback of such coils is the presence of a point at which the field actually departs so far from linearity that it reverses itself. This leads to the undesirable feature of aliasing which occurs wherever the object to be imaged extends beyond this point. To illustrate the undesirability of such effects in a medical imaging situation, aliasing would for example, cause part of the image, of a liver section to be superimposed upon the image of, say, a kidney, thereby obscuring valuable data. Another undesirable feature of previously employed transverse gradient coils is that they tend to concentrate the electric current into localized regions. This increases the inductance of the coils and reduces their ability to produce rapidly changing magnetic field gradients. It has been seen then that for NMR imaging purposes, coil inductance and resistance should be kept as low as possible.
Another transverse gradient coil design has been presented in a doctoral thesis submitted by Robert John Sutherland in which there is described a coil set designed around the assumption of an infinitely long cylinder. While such a design is theoretically valid, it is impractical in that even finite approximations are much longer than desired in NMR imaging or other applications. The design presented falls short in that satisfactory current return paths are not provided, either in fact or within the framework of the mathematical model employed.
Accordingly, it is seen that there is a need for a transverse gradient coil design based on distributed currents so as to lower the inductance and which is capable of producing transverse gradient fields of extreme linearity and which is practically sized for construction.